The invention relates, generally, to cryogenic liquid tanks and, more particularly, to an improved high temperature insulation for such tanks.
As is well known, cryogenic liquids, such as liquid natural gas (LNG), nitrogen, oxygen, CO.sub.2, methane, hydrogen and the like, are liquified gasses at extremely cold temperatures. Special problems are encountered in handling and storing cryogenic liquids because the cryogenic liquids will undergo phase changes at low temperature.
Cryogenic liquids are typically stored in tanks consisting of a vessel containing the liquid and a jacket spaced from and surrounding the vessel where the space between the vessel and jacket is filled with a thermal insulating media and is evacuated. This structure is typical of cryogenic storage tanks including large vertical tanks, small portable tanks and horizontal tanks found on LNG powered vehicles. Moreover, the large vertical storage tanks, which can be 35 feet tall, are supported on legs where industry standards require that the legs be insulated if they are greater than 18 inches high.
These elaborate insulating systems are required for a number of reasons. First, if the tanks are not thermally insulated, heat will be transferred to the cryogenic liquid causing it to vaporize and expand such that it must be vented from the tank resulting in wasted product. Moreover, the venting of the product may cause a hazard because many of these cryogens are extremely flammable. Finally, extremely fast vaporization of the cryogen fluid could cause the storage tank to burst creating a potential fire and explosion hazard.
One commonly used high performance thermal insulation is referred to as "super insulation" and consists of alternating layers of thin aluminum foil and glass fiber insulation. The super insulation is wrapped around the interior vessel and fills the evacuated space. While super insulation minimizes heat leak from the external environment to the cryogen liquid stored in a tank, it can degrade or even melt when subjected to high temperatures such as generated during a fire. This is even more important if the vacuum is lost and the insulation is subjected to a fire condition.
The degradation of the super insulation, resulting from exposure to high temperatures, will result in increased heat transfer to the liquid causing the "boil off" or vaporization of the cryogen liquid stored in the tank and resulting in venting of gas and/or the bursting of the tank or its associated plumbing. As will be apparent, the venting of vapor or the bursting of a tank of a cryogenic fluid like hydrogen in the area of a fire or other intense heat could result in a catastrophe.
While the insulating performance of other media, such as perlite, a granular insulating material, does not degrade when exposed to heat, these other forms of insulation do not have the superior insulating characteristics of super insulation and are not as desirable for thermally insulating cryogenic systems.
Thus, an insulation that has the insulating performance of super insulation that does not degrade when exposed to high temperature is desired.